1. Field of the Invention
The present invention relates to a display apparatus for receiving an image signal and carrying out an image display, and a picture display method in such a display apparatus.
2. Description of Related Art
As a display apparatus for picture display, a plasma display apparatus has been prevailing.
The principle of display operations in such a plasma display apparatus is the following, as is generally known. Gas is sealed in space formed, for example, by making two sheets of glass substrate face each other and surrounding it with a separation wall. Further, by applying a voltage on this gas, vacuum discharge is induced. Accordingly, within the space of the glass substrate, the gas is ionized and becomes at plasma state, thereby radiating ultraviolet rays. Here, if a fluorescent substance layer is preliminarily formed within the space between the glass substrates, the ultraviolet rays causes radiation of the visible light of a predetermined color by irradiating the fluorescent substance layer. The plasma display apparatus is configured to enable the display of colored picture by forming members corresponding to the three colors of R, G and B, as the fluorescent substance and display cells arranged in a matrix form or the like, and then achieving the above-mentioned discharge light emission phenomenon for each of the formed display cells.
One of display drive methods for the above-mentioned plasma display apparatus is a sub-field method.
The sub-field method is a drive method in which one field is divided into a plurality of sub-fields, and the light emission period of a display cell for each sub-field is controlled, thereby representing grayscale (brightness) of each display cell. Further, by controlling the grayscale of each display cell of R, G and B constituting single pixel, color reproduction of each pixel is achieved as well as the grayscale balance on the entire screen. In other words, color pictures may be represented.
Presently, the plasma display apparatus has low light emission efficiency at the time of display. For this reason, the case of displaying a bright image on the entire screen requires a considerably large amount of electric power. Accordingly, it is difficult to ignore an issue of an increase in an electric power consumption. Further, heat generation in circuits and display panel portion of the display apparatus is increased, thereby resulting decrease of reliability.
Accordingly, in the plasma display apparatus, so-called a PLE (Peak Luminance Enhancement) control is carried out when a picture is displayed. In the PLE control, firstly, for example, an average brightness level of image signals corresponding to the entire field screen is detected, and then a display brightness level, which is the brightness level to be used for actually carrying out the picture display, is set in accordance with the average brightness level. The drive operation based on, for example, the sub-field method is carried out so as to represent the grayscale corresponding to this set display brightness level.
The actual PLE control is designed so as to carry out the display of high brightness by setting the display brightness level high, if the average brightness level is low, even in the case of the signals having the same brightness level. On the contrary, if the average brightness level is high and bright, the display brightness level is set low, thereby limiting the electric power consumption.
Accordingly, the PLE control performed as mentioned above makes it possible to reduce the maximum electric power consumption, and further to enable display of images that are preferable in contrast.
As a display panel of the plasma display apparatus, a display panel with an aspect ratio of 16:9, which is longer in the lateral direction as compared with a standard aspect ratio of 4:3, is widely employed. When a picture having the aspect ratio of 4:3 is displayed on this display panel having the aspect ratio of 16:9, the picture having the aspect ratio of 4:3 is laterally expanded, in one method, to form a picture having the aspect ratio of 16:9.
However, if a picture having the aspect ratio of 4:3 is expanded to the picture having the aspect ratio of 16:9, the picture is laterally expanded and distorted for an amount corresponding to such an enlargement. In order to avoid such a distortion, a picture may alternatively be displayed, for example, as shown in FIG. 13A. In other words, the picture having the aspect ratio of 4:3 is displayed such that a picture area 201 is placed at a center in the lateral direction, on a display panel 200 having the aspect ratio of 16:9. In this case, no-picture regions on which a picture is not displayed are formed on both of right and left sides of the picture area 201. These regions are indicated as side panels 202, and designed so as to carry out the display, for example, by using a color and brightness close to black.
If such a display is utilized, although it results the side panel 202, the region on which no picture is displayed, the aspect ratio of 4:3 of the picture is maintained while the picture without any distortion is displayed.
Alternatively, as shown in FIG. 13B, a plurality of picture areas 201 may be set in the display panel 200. The side panel 202 may be set in regions outside the picture areas 201 within the display panel 200.
Strictly speaking, in view of the positional relation between the no-picture region and the picture areas 201 shown in FIG. 13B, the no-picture region may not be a side panel. However, in the present specification, if the no-picture region outside the picture area 201 within the display panel 200 is displayed, for example, in a color close to black, it is referred to as a side panel.
Further, as mentioned above, picture lights displayed in the plasma display apparatus is obtained from the visible light radiated from the fluorescent substance layers. However, it is known that this fluorescent substance layer is deteriorated in the course of use. Such a deterioration in the fluorescent substance is caused by the ultraviolet rays radiated by the vacuum discharge, impacts of ions generated within the vacuum space and the like.
Accordingly, the deterioration in the fluorescent substance is progressed as the accumulation time of the light emissions increases. In an actual display operation, the light emission accumulation times of the fluorescent substances corresponding to the respective display cells are not uniform, which causes variation on the basis of the pictures displayed up to now. In other words, the variation is induced in degrees of the deteriorations in the fluorescent substances among the display cells.
The deterioration in the fluorescent substance appears as an decrease of a light emission brightness. As mentioned above, the variation, which is induced in the degree of deterioration in the fluorescent substance corresponding to each display cell, results in variation of the light emission brightness of the fluorescent substance. For example, if the variation in the light emission brightness is induced among the fluorescent substances of R, G and B constituting one pixel, white balance may be disturbed.
Further, if the entire display screen is considered, there may be a case that a portion, which is a region that should be originally displayed with the same brightness and color as in its surrounding, may appear to have different brightness and color as the deterioration progresses. This is referred to as a so-called burn-in. If such a burn-in is generated, for example, a portion of the region in which the fluorescent substances are deteriorated may displays a fixed pattern and overlap with the original image. Accordingly, this is a known issue that causes deterioration of the display quality.
As a method of reducing the burn-in, for example, Japanese Patent Application Publication 2001-306026 was proposed. In this method, at first, it is judged whether a picture display by means of an input image signal is a moving picture display or a still picture display such as a fixed display of characters or the like. If it is judged as the moving picture display, for example, the usual PLE control is executed. However, if it is judged as the still picture display, the PLE control is not executed and a constant brightness display of a predetermined low brightness is carried out. According to the method described above, especially at the time of the still picture display, a difference in the brightness between the bright and dark regions of the picture is prevented to become larger. As a result, significant difference may not be generated in progressions of the deteriorations in the fluorescent substances, thereby reducing the burn-in.
Further, it is said that the burn-in is easily induced if a picture of fixed pattern with relatively higher contrast is displayed on the display panel for a long accumulation time. In other words, as compared with the fluorescent substance on a picture portion of darker display, the fluorescent substance of brighter picture portion has a longer light emission accumulation time. Accordingly, the degree of deterioration in the fluorescent substance differs greatly between the bright display region and the dark display region, thereby generating a burn-in with a clearly viewable boundary.
Accordingly, in the plasma display apparatus in which the side panel 202 is set in the display region as previously shown in FIGS. 13A, 13B, the burn-in may be easily induced in a boundary between the picture area 201 and the side panel 202.
Therefore, the plasma display apparatus is required to reduce such burn-in in order to deal with the case in which a picture is displayed together with the side panel 202. To do so, for example, it is also possible to apply the above described method for reducing the burn-in by turning the PLE control on or off in accordance with the decision result on whether the displayed picture is a moving picture display or a still picture display. If this method is applied, boundary between the picture area 201 and the side panel 202 is detected as the still picture, and the PLE control is turned off, and the constant low brightness is used for the display. However, for example, this may cause the display brightness in the picture area 201 to be dropped to a certain degree. In other words, the region of the side panel 202 has a brightness substantially equal to all black. If the display brightness of the picture area 201 is decreased, the progress of the deterioration in the fluorescent substance of the picture area 201 may be delayed, thereby delaying the progress of the burn-in as a result.
However, in this case, since the picture displayed on the picture area 201 is made darker, the image quality is deteriorated and it is not preferable.
Accordingly, in the present situation, the following method is widely employed on the contrary to the above. In the method, a certain degree of brightness is given to the side panel 202 which is in gray state under the PLE control that is normally executed. In this case, the deterioration in the fluorescent substance of the panel portion corresponding to the side panel 202 progresses in such a way that no large difference is generated between the progress of the deterioration in the fluorescent substance of the panel portion corresponding to the picture area 201 and that of the side panel, thereby delaying the progress of the burn-in.